Everyone believes the US will leave at the latest by the end of Bush's term, and knows that as soon as we do, the war to control Iraq begins all over again, this time without Queensbury rules. Most of the Shia recognized that the largest power in Iraq as soon as that happens will be Iran, and its clients, the Sadr wing of the Shia.
The only forces inside Iraq willing to fight Sadr, that being the case, are the Sunnis. They expect to be supported with gobs of money by the Saudis. Saudi Arabia and Iran will conduct a proxy war through their clients as soon as the US leaves. Iran is almost certain to win that war.
Petraeus has recently been arming the Sunnis as a means of trying to pressure Maliki, but it will only drive him deeper in the arms of Iran. Maliki is emphatically preparing himself and his party for the time when your conclusion, that Iraq isn't worth it, is acted upon. The result will be first a civil war on a higher level of conflict, second the defeat of the Sunnis without restraint in how they are treated as they lose (which will drive millions into exile in Jordan and Saudi Arabia), and finally Iranian alliance with the new Iraq.
OK, neither are the Shi'ites. It's impossible to choose sides in this. I've fought in Sadr City (all Shi'ite) and Dora (vast majority Sunni). We've taken way too many casualities in both areas. So which side do I crack down on? I was clearing an apartment complex that was majority Shi'a once. We met a very nice Sunni family that were terrified (and under direct intimidation) to leave their apartment. The mother worked in the Green Zone, but she couldn't go anywhere due to the very real threats of her whole family being killed if she did. I cleared another Sunni neighborhood where I met a couple of Shi'a families who were hightailing it for the South due to Sunni intimidation and violence.
If that's not all bad enough, we have the Kurds up north who really, really like us. The Turks really, really hate them and are supposedly allies with us (to say nothing of anti-Kurdish violence with Syria and Iran, too). Do we help them and alienate Turkey? Do we ignore them and possibly allow them to be wiped out by Turkey, Syria, and Iran?
The point is, there are WAY too many variables for us to be terribly effective in the region. We just need to boil it down to the essentials. We're involved in the Middle East for oil. Bottom line. Why don't we seize those fields and take it? We've paid more than enough for it, I'd say.